Tuesday, June 18, 2013

Emergency Lamp Using Trasnistor As Switch

Good morning everyone, let's build an emergency lamp that will light up using a battery as soon as supply voltage drops. The main idea for this post is mostly to see how transistors can be used as switches. It will also give you an idea of NOT gate too.

Components needed:

1. Power supply unit.
2. Transistor, will be using NPN transistors.
3. Resistors, values can vary.
4. Switch, single pole single throw will suffice.
5. Battery.
6. Load can be a small lamp depends on battery and capability of driver transistor. 

For ease of understanding it is better to break down the whole circuit into couple different segments.

Segment 1: The power supply unit.

Simple AC to DC power supply.

For this project a simple power supply will be enough as we won't be using that much power. This simple circuit that is shown above simply provides a regulated DC output from the AC wall outlet. The way it works is pretty simple. First step is the transformer which in this case is being used as step down so we are simply lowering the voltage. This low voltage AC is converted to DC using four general purpose diode. This voltage is smoothed using capacitors and being regulated using a regulator.

Note: You might have to change the values of smoothing capacitor if you want to charge high capacity battery. A heat sink might be necessary for 7812 if current draw exceeds 400mA.

Segment 2: Auto turn on circuit.

Working principle of this circuit is very simple. Here a pair of Transistors is being used as switches so that this circuit does not charge or try to charge the battery. If we start analyzing from the left we can see the first transistor from the left has two resistors with it's base. When the AC power is available we have a voltage difference in A and B. As it is a NPN transistor and from the resistor values it is evident that this transistor will turn on. As soon as it turns on it will pull down the base of the second transistor to ground even if the switch is turned on as current through the resistor with the switch will reach the ground point via the left transistor. So the right one will be turned off, so does the load.

But when the power at the AC outlet fails, there will be no positive voltage at A, which means the base of the left transistor will be pulled down to ground by the resistor that is between it's base and ground. So this transistor will turn off. As soon as left transistor turns off the right one's base is pulled up by the resistor connected to it's base via the switch, so the transistor on the right will turn on and so does the load.

That means if main power fails the load will automatically turn on using the power stored in the battery. Like I mentioned above it is pretty much a NOT gate, I guess now you have a better idea.

Segment 3: Typical load.

A Simple LED array can be used.

LED array with multiple Resistor to control Brightness

Here is a simple LED array with multiple Resistors for brightness control.Use a three way selector switch to switch between different values. Remember the resistance and wattage rating of the resistors will depend on your battery and number of LED on the array.

Segment 4: Using rechargeable battery.

The reason I haven't included the charging circuit here is because that circuit will change depending on the type of battery used. So to keep things simple I have just shown how to connect the charger. In later posts I will post different charging circuit for different battery types.

In the diagram I have marked different points of connection so that you will easily get the bigger picture.


1. Transistor Datasheet.
2. AC to DC power supply circuit.
3. Linear voltage regulator.
4. Visit my blog.

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